1. Field of the Invention
This invention relates new to aqueous polyurethane dispersion adhesive compositions which can be an one component or a two component for use in thermoforming. More particularly, this invention relates to aqueous polyurethane dispersion adhesive compositions comprising mixed sulfonate and carboxylate anionic polyurethane dispersions, and with or without compatible crosslinking agents. These new adhesive compositions exhibit good stability at low pH values, long pot life, high green strength, good water resistance and improved heat resistance.
2. Description of the Prior Art
Aqueous dispersions of polyurethanes are known and are described in, for example, U.S. Pat. No. 3,479,310; Angew. Chem., 82, 53, (1972); and Angew. Makromol. Chem., 98, (1981).
U.S. Pat. No. 3,479,310 to Dieterich et al discloses dispersing in water a fully chain extended, NCO-free polyurethane having internal ionic salt groups.
For anionic aqueous polyurethane dispersions, the ionic salt groups are salts of carboxylic acid or sulfonic acid.
Aqueous polyurethanes having only internal carboxylate groups as anionic moieties are stable only at pH&gt;7. Such polyurethane dispersions can form a two component adhesive composition with compatible crosslinkers, such as polyfunctional isocyanates, epoxies or aziridines.
Henning, et al., U.S. Pat. No. 4,870,129, discloses use of the sodium salt of N-(2-aminoethyl)-2-aminoethane sulfonic acid (AAS salt) to prepare polyurethane dispersions. The aqueous polyurethane dispersions are reported to have exhibited good stability at low pH values (5-7), high green strength and medium heat resistance.
Usually, after blending the sulfonated polyurethane dispersions with a polyisocyanate crosslinker, such as the dispersible polyisocyanate in U.S. Pat. No. 4,663,377 to Hombach et al, their heat resistance will be improved.
Leung, U.S. Pat. No. 4,762,880, discloses water-based thermoforming adhesives comprising aromatic polyurethanes, cross-linking agents and others. These kind of adhesive compositions will need high temperature to activate due to the aromatic polyurethane component.
U.S. Pat. No. 4,870,129 to Henning et al discloses an adhesive consisting of an aqueous polyurethane containing chemically incorporated carboxylate or sulfonate groups. The adhesive of this reference showed low activation temperature but only medium heat resistance.
Duan et al in copending application Ser. No. 08/304,653, filed Sep. 9, 1994, and copending application 08/343,676, filed Nov. 22, 1994 as a continuation in part of 08/126,508, filed Sep. 24, 1993, now abandoned, disclose aqueous polyurethane dispersions based on sulfonated polyester polyols, which have an unusually high crystallization rate, while also exhibiting good stability at low pH values, high green strength, and medium and high heat resistance.
In the preparation of anionic polyurethane dispersions it is generally preferred to prepare a polyurethane prepolymer having a small residual free isocyanate content, disperse the prepolymer in water, and then add a plural functional relatively low molecular weight primary and/or secondary amine as a chain extender. This chain extension process is needed because a higher molecular weight polyurethane-urea having high heat resistance is obtained after extension. Chain extension, however presents some problems. The reaction of an amine with the free isocyanate of the prepolymer is a very rapid and vigorous reaction, therefore increasing the possibility of side reactions (creating a branching or network structure) and gelling.
It is known that chain extension can also be accomplished by permitting reaction of an isocyanate functional group on the polyurethane prepolymer with water via a mechanism which is believed to generate amine functional group on the prepolymer which promptly reacts with another isocyanate functional group of the prepolymer to give a self-extended polymer. However, carbon dioxide is given off in this reaction and the pH of the dispersion consequently drops during the extension reaction. For carboxylate anion dispersions, the pH drop can cause the polymer to come out of dispersion. For sulfonate dispersions, such as disclosed in the examples of GB 1,128,568, high amounts of solvent are used to accomplish dispersion (approximately 1/3 of the total dispersion weight), most of which is subsequently distilled off. A stable dispersion reportedly results. However the equipment needed to distill off and recover excess solvent is very expensive and not practical for producing polyurethane dispersion products with optimal properties for specific industrial applications.